For internal combustion engines, such as diesel engines, nitrogen oxide (NOx) compounds may be emitted in the exhaust. To reduce NOx emissions, a selective catalytic reduction (SCR) process may be implemented to convert the NOx compounds into more neutral compounds, such as diatomic nitrogen, water, or carbon dioxide, with the aid of a catalyst and a reductant. The catalyst may be included in a catalyst chamber of an exhaust system, such as that of a vehicle or power generation unit. A reductant, such as anhydrous ammonia, aqueous ammonia, or urea may be typically introduced into the exhaust gas flow prior to the catalyst chamber. To introduce the reductant into the exhaust gas flow for the SCR process, an SCR system may dose or otherwise introduce the reductant through a dosing module (doser) that vaporizes or sprays the reductant into an exhaust pipe of the exhaust system upstream of the catalyst chamber. The SCR system may include one or more sensors to monitor conditions within the exhaust system. Further, the SCR system may include an oxidation catalyst, for example a diesel oxidation catalyst (DOC), to oxidize hydrocarbons and carbon monoxide in the exhaust gas and/or a particulate filter, for example a diesel particulate filter (DPF), to remove diesel particulate matter or soot from the exhaust gas of a diesel engine.
Catalysts such as the DOC and DPF that contain platinum-group metals (PGM), which help oxidize soot, hydrocarbon, and nitric oxide (NO), can also alter the ratio of nitric oxide and nitrogen dioxide (NO2) that enters a downstream SCR catalyst. It is useful to diagnose the capability of PGM-containing catalysts to alter the SCR feedgas NO and NO2 content: for an SCR to perform well, on its upstream, the DOC needs to oxidize a portion of NO to NO2; thus, a DOC that is not capable of oxidizing enough NO may not be able to provide quality feedgas containing NO2 to the SCR to reduce NOx emissions.